Ball joints are used in a wide range of applications. For example, ball joints are widely used in universal joints employed in the suspension devices or steering devices of automobiles, or the like. Furthermore, ball joints are also used in the joints between pistons and shoes in a hydraulic swash plate piston pump.
A hydraulic swash plate piston pump employs either a ball joint wherein a piston has a ball section and a shoe has a socket which accommodates that ball section, or conversely, a ball joint wherein the shoe has a ball section and the piston has a socket.
Patent Citation 1 discloses reducing the diameter of a wall surrounding the opening of a socket by a caulking process, in a state where a ball section is accommodated in the socket, in the manufacture of a ball joint. As a specific method for a caulking process of this kind, Patent Citation 2 discloses reducing the diameter of a wall surrounding the opening of a socket simultaneously about the whole circumference thereof, by inserting the socket into a cylindrical metal die and pressing the socket against the die in the axial direction.